Nitride semiconductor device having a silicon-containing layer and manufacturing method thereof

ABSTRACT

A semiconductor device and a manufacturing method thereof are provided which enable reduction and enhanced stability of contact resistance between the back surface of a nitride substrate and an electrode formed thereover. A nitride semiconductor device includes an n-type Ga—N substrate ( 1 ) over which a semiconductor element is formed, and an n-electrode ( 10 ) as a metal electrode formed over the back surface of the GaN substrate ( 1 ). A connection layer ( 20 ) is formed between the GaN substrate ( 1 ) and the n-electrode ( 10 ), and the connection layer ( 2 ) is composed of a material that is other than nitride semiconductors and that contains silicon.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nitride semiconductor device and amanufacturing method thereof, and particularly to a nitridesemiconductor device having an n-electrode provided over the backsurface of a nitride semiconductor substrate.

2. Description of the Background Art

As devices like blue light-emitting diodes (LEDs) and blue laser diodes(LDs) are put in practical use, nitride semiconductor devices using,e.g., gallium nitride (GaN), indium gallium nitride (InGaN), andaluminum gallium nitride (AlGaN), are attracting increased attention.

Usually, the crystals of nitride semiconductors, such as GaN, havehexagonal wurtzite structure. Accordingly, a nitride semiconductorsubstrate formed through crystal growth has a polarity, and so has afront surface called Ga surface and a back surface called N surface.

For the crystal growth of nitride semiconductors, it is known that thegrowth on the front surface (Ga surface) of a nitride semiconductorsubstrate offers good crystal quality. Accordingly, in the manufactureof nitride semiconductor devices, the layered-structure of thesemiconductor element is usually formed by crystal growth of nitridesover the front surface of the nitride semiconductor substrate.Accordingly, in a laser diode as a conventional nitride semiconductordevice, both of its n-electrode and p-electrode are formed over thefront surface of the substrate. However, the manufacturing process thenrequires removing part of the layered-structure of the semiconductorelement in order to expose the surface of the substrate, and thiscomplicates the manufacturing process. Also, providing two electrodes onone side (on the front surface side) of the substrate requires a deviceformation area that is about two times larger than the area requiredwhen one electrode is formed on each side, which hinders miniaturizationof the semiconductor device.

Accordingly, nitride semiconductor devices having electrodes on bothsides are recently developed in which the n-electrode is formed over theback surface of the nitride semiconductor substrate (for example, seeJapanese Patent Application Laid-Open Nos. 2004-71657 and 11-340571(1999), which are hereinafter referred to respectively as PatentDocument 1 and Patent Document 2).

Patent Document 2 discloses a technique in which, during a process offabricating a laser diode, a GaN layer (n-contact layer) doped withsilicon is grown on the back surface of a GaN substrate, and the n-typeelectrode is formed on the GaN layer.

On the back surface (N surface) of the nitride semiconductor substrate,deposited metal is more likely to exfoliate than on the front surface(Ga surface). Accordingly, a metal electrode formed on the back surfaceof the substrate tends to exhibit larger resistance (contact resistance)between the electrode and the substrate. For example, in a laser diode,if the contact resistance of the n-electrode is not sufficiently low, anincreased voltage (operating voltage) is required to cause the laserdiode to operate, and electric characteristics vary due to thegeneration of heat during operation. This makes it difficult to obtainstable output in the prescribed temperature range. It is thereforedesirable to further reduce the contact resistance of the n-electrodeprovided over the back surface of the nitride semiconductor substrate.

When, as described in Patent Document 2, a silicon-doped GaN layer isgrown on the back surface of a GaN substrate and an n-electrode isformed on the GaN layer, the n-electrode offers enhanced ohmicproperties and enhanced adhesion as compared with conventional ones.However, when a thermal treatment is performed after the formation ofthe n-electrode, the reaction between the GaN layer surface and then-electrode progresses and then the electric characteristics at theinterface between the GaN layer and the n-electrode may be deteriorated,depending on the conditions of the thermal treatment (for example, thebarrier height is increased (the tunneling is reduced)).

SUMMARY OF THE INVENTION

An object of the present invention is to provide a nitride semiconductordevice and a manufacturing method thereof which enable reduction andenhanced stability of contact resistance between the back surface of anitride substrate and an electrode formed thereover.

According to the present invention, a nitride semiconductor deviceincludes: a nitride semiconductor substrate; a layered-structure of anitride semiconductor element that is formed over a first main surfaceof the nitride semiconductor substrate; a metal electrode disposed overa second main surface of the nitride semiconductor substrate; and aconnection layer formed between the nitride semiconductor substrate andthe metal electrode. The connection layer is composed of a material thatis other than nitride semiconductors and that contains silicon.

It is possible to stably maintain small contact resistance even whenthermal treatment is performed after the formation of the metalelectrode. Accordingly, the contact resistance of the metal electrode isnot increased even through temperature variations in the devicefabrication. That is, the contact resistance between the nitridesemiconductor substrate and the metal electrode is kept small even afterthe completion of device fabrication. It is thus possible to reduce theoperating voltage of the nitride semiconductor device and to lesseninfluences of heat generation, thus providing stable operating outputand enabling high-power output.

According to the present invention, a method of manufacturing a nitridesemiconductor device includes the following steps (a) to (d). The step(a) forms a layered-structure of a nitride semiconductor element over afirst main surface of a nitride semiconductor substrate. The step (b)forms a connection layer over a second main surface of the nitridesemiconductor substrate, the connection layer being composed of amaterial that is other than nitride semiconductors and that containssilicon. The step (c) forms a metal electrode on the connection layer.The step (d) performs a thermal treatment after the step (c).

The presence of the connection layer between the nitride semiconductorsubstrate and the metal electrode prevents the reaction between themetal electrode and the nitride semiconductor substrate during thermaltreatment performed after the formation of the metal electrode. Thisprevents reduction of carrier concentration in the second main surfaceof the nitride semiconductor substrate that would be caused by thereaction, and prevents the increase of barrier height (the reduction oftunneling) between the nitride semiconductor substrate and the metalelectrode. Thus, even when thermal treatment is performed, it ispossible to stably maintain small contact resistance between the metalelectrode and the nitride semiconductor substrate.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an example of the structure of a nitridesemiconductor device according to a first preferred embodiment;

FIG. 2 is an enlarged cross-sectional view showing the back surface sideof the substrate of the nitride semiconductor device of the firstpreferred embodiment;

FIGS. 3 to 7 are process diagrams showing a method of manufacturing thenitride semiconductor device of the first preferred embodiment;

FIG. 8 is a graph used to describe an effect of the first preferredembodiment;

FIG. 9 is an enlarged cross-sectional view showing the back surface sideof the substrate of a nitride semiconductor device according to a secondpreferred embodiment;

FIGS. 10 and 11 are process diagrams showing a method of manufacturingthe nitride semiconductor device of the second preferred embodiment; and

FIG. 12 is a diagram showing a modification of the nitride semiconductordevice of the second preferred embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Preferred Embodiment

FIG. 1 is a diagram showing an example of the structure of a laser diodeas a nitride semiconductor device according to a first preferredembodiment of the present invention. As shown in FIG. 1, thesemiconductor device uses an n-type GaN substrate 1 as the nitridesemiconductor substrate.

On the front surface (a first main surface), or the Ga surface, of theGaN substrate 1, an n-type AlGaN cladding layer 2, an n-type GaN guidelayer 3, an active layer 4, a p-type GaN guide layer 5, a p-type AlGaNcladding layer 6, and a p-type GaN contact layer 7 are formed as alayered-structure of nitride semiconductors, and the GaN substrate 1 andthe layered-structure form a laser diode device (a nitride semiconductorelement). A p-electrode 8 is formed on the p-type GaN contact layer 7.The p-type AlGaN cladding layer 6 and the p-type GaN contact layer 7 arepatterned in a given shape by etching. The p-electrode 8 is disposed ontop of the p-type GaN contact layer 7. Also, an SiO₂ film 9, serving asa protective film, is formed to cover the upper part of the nitridesemiconductor device, with the top surface of the p-electrode 8 exposed.

An n-electrode 10 is formed as a metal electrode over the back surface(a second main surface), or the N surface, of the GaN substrate 1. Inthis preferred embodiment, a connection layer 20 that is made of acertain material other than nitride semiconductors is provided betweenthe n-electrode 10 and the GaN substrate 1.

The connection layer 20 is capable of electrically connecting the GaNsubstrate 1 and the n-electrode 10, and preferably, it causes no voltageloss. Also, the connection layer 20 is capable of stably offering goodelectric characteristics even when it experiences thermal treatment.Accordingly, it is desired that the connection layer 20 have a filmthickness of 5 nm or less and be formed uniformly. When the connectionlayer 20 is too thick, it works as a resistance layer to cause increasedvoltage loss, and therefore it is preferable to form the connectionlayer 20 thin in a range in which the effects of the invention(described later) can be obtained.

In this preferred embodiment, the connection layer 20 is composed of amaterial that contains silicon. Specific examples thereof includehexamethyldisilazane (HMDS), which is an organic-silicon-based material,and siloxane-based or TEOS-based organic materials.

FIG. 2 is an enlarged cross-sectional view showing the back surface sideof the GaN substrate 1 of the nitride semiconductor device shown inFIG. 1. FIG. 2 shows the GaN substrate 1 with its back surface facingup, i.e., the GaN substrate 1 of FIG. 1 is turned upside down in FIG. 2.

As mentioned above, the connection layer 20 lies between the backsurface of the GaN substrate 1 and the n-electrode 10. The n-electrode10 has a three-layered structure including a Ti film 11, a Pt film 12,and an Au film 13, and the Ti film 11 forms the surface that connectswith the connection layer 20. This offers good ohmic properties betweenthe n-electrode 10 and the GaN substrate 1.

FIGS. 3 to 7 are process diagrams illustrating a process ofmanufacturing the nitride semiconductor device of this preferredembodiment, and the diagrams particularly show the process steps forforming the electrode structure on the back side of the GaN substrate 1shown in FIG. 2. The process of forming the n-type AlGaN cladding layer2, the n-type GaN guide layer 3, the active layer 4, the p-type GaNguide layer 5, the p-type AlGaN cladding layer 6, the p-type GaN contactlayer 7, the p-electrode 8, and the SiO₂ film 9 over the front surfaceof the GaN substrate 1 shown in FIG. 1 is not described in detailherein, because they can be formed by conventional methods (for example,a method described in Patent Document 1 or 2).

In the process of manufacturing the nitride semiconductor device of thispreferred embodiment, given pretreatment processes are applied to theback surface of the GaN substrate 1, prior to the formation of then-electrode 10 (FIG. 3). The pretreatment processes include polishingand grinding for thinning of the GaN substrate 1 and removal of damagelayer, dry-etching for nitrogen defect formation, oxygen plasmatreatment and BHF treatment for contamination removal, and the like.

For a specific example of the polishing and grinding process, the backsurface of the GaN substrate 1 is first ground by about 100 to 200 μmwith a grinding machine, next the ground surface is made flat withdiamond slurry, and then it is polished with abrasive cloth usingalumina as abrasive, for example. The dry-etching process may useInductively Coupled Plasma (ICP), Reactive Ion Etching (RIE), orElectron Cyclotron Resonance (ECR). Chlorine (Cl)-based gas is used asthe etching gas.

The oxygen plasma treatment is capable of removing surface contaminationsource of carbon, and the introduction of defects into the back layer ofthe GaN substrate 1 by the oxygen plasma treatment offers the effect ofincreasing the carrier concentration in a pseudo manner. The oxide-basedlayer formed on the back surface of the GaN substrate 1 due to theoxygen plasma treatment may be removed by BHF treatment when needed.

After the pretreatment has been finished, the connection layer 20 isformed on the back surface of the GaN substrate 1 (FIG. 4). Theconnection layer 20 may be formed by applying a solution that containsconstituent material of the connection layer 20 onto the substrate 1, orby exposing it in an atmosphere that contains constituent material ofthe connection layer 20, for example.

After the formation of the connection layer 20, the Ti film 11, the Ptfilm 12, and the Au film 13, for forming the n-electrode 10, aresequentially deposited over the back surface of the GaN substrate 1 by,e.g., Electron Beam (EB) evaporation (FIG. 5). The n-electrode 10 havingthe three-layered structure of Ti/Pt/Au is thus formed. The filmthickness of the Ti film 11 can be about 10 to 100 nm. The Pt film 12 isformed to such a thickness as to obtain a barrier effect to prevent thereaction between the Ti film 11 and the Au film 13 in following thermaltreatment, specifically to a thickness of about 50 to 100 nm. The Aufilm 13 requires such a film thickness that it will not be removed dueto the reaction with solder during the device fabrication, andspecifically a film thickness of about 200 nm or more is enough, forexample.

The n-electrode 10 is patterned as needed. That is, as shown in FIG. 6,a predeterminedly-patterned resist mask 50 is formed on the n-electrode10, and unwanted part of the n-electrode 10 is removed by using themask, e.g., by wet-etching or ion-milling (FIG. 7). The resist mask 50is then removed.

The formation of the n-electrode 10 is followed by a thermal treatment.In this preferred embodiment, the n-electrode 10 is formed over the backsurface of the GaN substrate 1 with the connection layer 20 sandwichedtherebetween, and therefore the reaction between the n-electrode 10 andthe GaN substrate 1 is prevented during the thermal treatment. Thisprevents the reduction of carrier concentration in the back layer of theGaN substrate 1 due to the reaction, and prevents the increase ofbarrier height (the reduction of tunneling) between the GaN substrate 1and the n-electrode 10. Thus, even when thermal treatment is performed,it is possible to stably maintain small contact resistance between then-electrode 10 and the GaN substrate 1.

This thermal treatment uses a temperature not lower than those used inthe following device fabrication. The thermal treatment atmosphere maybe of any of air, nitrogen, oxygen, and inert-gas. This thermaltreatment stabilizes the contact resistance between the GaN substrate 1and the n-electrode 10. It is desired that the contact resistance remainstable even through thermal treatment at 160° C. or higher, and it ismore desired that it remain stable even through thermal treatment at400° C. or higher.

The n-electrode structure of this preferred embodiment is thus formed onthe back surface of the GaN substrate 1 through the process stepsdescribed above.

FIG. 8 is a graph used to describe the effect of the first preferredembodiment, which shows a relation between the temperature of thermaltreatment performed after the formation of the n-electrode of a nitridesemiconductor device and the contact resistance between the n-type GaNsubstrate and the n-electrode. The graph shows a comparison between aconventionally-structured device (i.e., a structure not having theconnection layer 20 between the GaN substrate 1 and the n-electrode ofFIG. 1) and the device of the first preferred embodiment. Then-electrode was made as a three-layered structure of Ti/Pt/Au in bothdevices. The values of contact resistance shown on the vertical axis inthe graph are relative values that are obtained assuming that theconventional structure exhibits a contact resistance of “1” when notsubjected to thermal treatment.

As shown in FIG. 8, in the conventionally-structured nitridesemiconductor device, the contact resistance between the n-type GaNsubstrate and the n-electrode increases as the thermal treatmenttemperature becomes higher over 160° C. On the other hand, in thestructure having the connection layer 20 of this preferred embodiment,the contact resistance between the n-type GaN substrate and then-electrode is kept nearly fixed even when the thermal treatmenttemperature is increased. That is, the n-electrode of the preferredembodiment exhibits small contact resistance, and the contact resistanceremains stable without depending on the thermal treatment temperature.

That is, according to this preferred embodiment, it is possible tostably maintain small contact resistance even when thermal treatment isperformed after the formation of the n-electrode 10. The contactresistance of the n-electrode 10 is thus not increased even throughtemperature variations in the device fabrication. That is, the contactresistance between the GaN substrate 1 and the n-electrode 10 is keptsmall even after the completion of the device fabrication. It is thuspossible to reduce the operating voltage of the nitride semiconductordevice and to lessen influences of heat generation, which providesstable operating output and enables high-power output.

Also, the adhesion of the n-electrode 10 is enhanced because then-electrode 10 made of metal is positioned not directly on the backsurface of the GaN substrate 1 but with the connection layer 20sandwiched therebetween. Furthermore, it is possible to enlarge thetemperature margin in thermal treatment because the contact resistanceof the n-electrode is stable and does not depend on the thermaltreatment temperature.

This preferred embodiment has shown an example in which the n-electrode10 has a three-layered structure of Ti/Pt/Au, but the same effects areobtained with a two-layered structure of Ti/Al or a two-layeredstructure of Ti/Au, for example (in both cases, the Ti layer is placedin contact with the connection layer 20). Other possible structures ofthe n-electrode include single-layered or multi-layered structures inwhich a material having a work function of 5 eV or less, e.g., Al, Ta,TiN, etc., is placed in contact with the connection layer 20.

Also, the example above uses an n-type GaN substrate as a nitridesemiconductor substrate, but this preferred embodiment is applicablealso to nitride semiconductor layers of gallium nitride (GaN), indiumgallium nitride (InGaN), aluminum gallium nitride (AlGaN), etc.

Second Preferred Embodiment

FIG. 9 is an enlarged cross-sectional view showing the back surface sideof the GaN substrate 1 of a nitride semiconductor device according to asecond preferred embodiment. In FIG. 9, the same components as those ofFIG. 2 are shown at the same reference characters.

As shown in FIG. 9, the nitride semiconductor device of this preferredembodiment has an oxide film 21 that is thin (hereinafter referred to as“a thin oxide film”) between the back surface of the GaN substrate 1 andthe connection layer 20 (i.e., at the interface between the GaNsubstrate 1 and the connection layer 20). In other respects, the nitridesemiconductor device is structured the same as the nitride semiconductordevice of the first preferred embodiment (described with FIGS. 1 and 2).

Providing the thin oxide film 21 at the interface between the GaNsubstrate 1 and the connection layer 20 enhances the adhesion betweenthe GaN substrate 1 and the connection layer 20, i.e., the adhesionbetween the GaN substrate 1 and the n-electrode 10, in addition to theeffects described in the first preferred embodiment. It is necessarythat the thin oxide film 21 not electrically insulate the n-electrode 10and the GaN substrate 1 from each other, since the n-electrode 10 has tobe electrically connected to the GaN substrate 1. For example, when thethin oxide film 21 is formed to a thickness of 1 nm or less, it does notfunction as a so-called “insulating film”, but it allows passage ofcurrent through the thin oxide film 21, and then the electric connectionbetween the n-electrode 10 and the GaN substrate 1 is ensured. It is notnecessary to form the thin oxide film 21 with a uniform thickness,because the purpose of the formation of the thin oxide film 21 is notinsulation.

Also, instead of the thin oxide film 21 formed in the entire area of theinterface between the GaN substrate 1 and the connection layer 20, thethin oxide film 21 may be formed partially (like islands, for example),e.g., as shown in FIG. 12, for the purpose of ensuring electricconnection between the n-electrode 10 and the GaN substrate 1 (in thiscase, too, a thickness of 1 nm or less is desirable). In view of theohmic properties between the n-electrode 10 and the GaN substrate 1, itis rather preferable to partially form the thin oxide film 21.

According to this preferred embodiment, the adhesion between the GaNsubstrate 1 and the n-electrode 10 is enhanced, and so the contactresistance between the GaN substrate 1 and the n-electrode 10 is furtherreduced. This allows reduction of the operating voltage of the nitridesemiconductor device, reduces influences of heat generation, andprovides stable operating output and enables high-power output.

A method of manufacturing the nitride semiconductor device of thispreferred embodiment, particularly process steps for forming theelectrode structure on the back surface of the GaN substrate 1 shown inFIG. 9, will be described below.

In this preferred embodiment, as in the first preferred embodiment,given pretreatment processes are applied to the back surface of the GaNsubstrate 1 prior to the formation of the n-electrode 10, and the thinoxide film 21 is formed during the pretreatment (FIG. 10). The thinoxide film 21 may be formed by exposing the back surface of the GaNsubstrate 1 in an atmosphere of oxygen, by exposing it in an atmosphereof oxygen plasma or oxygen radical, or by performing chemical treatmentusing an oxidizing chemical (e.g., oxidation with a mixed solution ofH₂SiF₆ and H₃BO₃, or anodic oxidation with N-methylacetamide), forexample.

It is desired that the thin oxide film 21 formed in this process have athickness of 1 nm or less, in order to ensure electric connectionbetween the n-electrode 10 and the GaN substrate 1. Alternatively, thethin oxide film 21 may once be formed somewhat thick and then thinned to1 nm or less by BHF treatment. Or, the thin oxide film 21 may be onceformed on the entire back surface of the GaN substrate 1 and thenpartially removed by BHF treatment such that it partially remains asshown in FIG. 12.

Then, with the thin oxide film 21 remaining on the back surface of theGaN substrate 1, the connection layer 20 is formed thereon (FIG. 11).The connection layer 20 may be formed by applying a solution thatcontains constituent material of the connection layer 20 onto thesubstrate 1, or by exposing it in an atmosphere that containsconstituent material of the connection layer 20, for example.

After that, as described in the first preferred embodiment, then-electrode 10 is formed, and patterned as needed, and given thermaltreatment is performed, so as to complete the n-electrode structure ofthis preferred embodiment on the back surface of the GaN substrate 1.

While the invention has been described in detail, the foregoingdescription is in all aspects illustrative and not restrictive. It isunderstood that numerous other modifications and variations can bedevised without departing from the scope of the invention.

1. A method of manufacturing a nitride semiconductor device, comprising: (a) forming a layered-structure of a nitride semiconductor element over a first main surface of a nitride semiconductor substrate; (b) forming a connection layer over a second main surface of said nitride semiconductor substrate, said connection layer being composed of a material that is other than nitride semiconductors and that contains silicon; (c) forming a metal electrode on said connection layer; and (d) after (c), performing a thermal treatment.
 2. The nitride semiconductor device manufacturing method according to claim 1, further comprising (e): applying an oxygen plasma treatment to said second main surface of said nitride semiconductor substrate.
 3. The nitride semiconductor device manufacturing method according to claim 2, further comprising (f): after said step (e), applying a BHF (Buffered Hydrofluoric Acid) treatment to said second main surface of said nitride semiconductor substrate.
 4. The nitride semiconductor device manufacturing method according to claim 1, further comprising (g): forming an oxide film on said second main surface of said nitride semiconductor substrate, wherein said (b) is performed with said oxide film remaining on said second main surface of said nitride semiconductor substrate.
 5. The nitride semiconductor device manufacturing method according to claim 4, wherein said (g) is achieved by applying at least one of an oxygen plasma treatment, a treatment of exposing in an atmosphere of oxygen radical, and a chemical treatment using a given chemical having an oxidizing action, to said second main surface of said nitride semiconductor substrate.
 6. The nitride semiconductor device manufacturing method according to claim 4, further comprising (h): partially removing said oxide film formed in said (g).
 7. The nitride semiconductor device manufacturing method according to claim 6, wherein said (h) is achieved by a BHF treatment.
 8. The nitride semiconductor device manufacturing method according to claim 1, wherein said (b) is achieved by applying a solution that comprises constituent material of said connection layer to said second main surface, or by exposing said second main surface in an atmosphere that comprises constituent material of said connection layer, or by depositing constituent material of said connection layer over said second main surface.
 9. The nitride semiconductor device manufacturing method according to claim 1, wherein the material of said connection layer is an organic material.
 10. The nitride semiconductor device manufacturing method according to claim 1, wherein the electrode comprises titanium (Ti). 